The present invention generally relates to an image device such as a television receiver equipped with a CRT (cathode-ray tube). More specifically, the present invention relates to a shading correction circuit of a television receiver capable of correcting brightness inequalities.
In conventional image devices, pictures are displayed on display devices. For instance, in an image-receiving tube (referred to as a "CRT" hereinafter) of a television receiver, three-color (red (R), green (G), blue (B)) electron guns are arranged in a neck portion of a funnel-shaped glass bulb, and an anode and a fluorescent screen are provided in a cone-shaped portion of this glass bulb. Electron beams emitted from the respective R, G, B-electron guns are accelerated by an anode voltage of a high voltage so as to impinge onto the fluorescent screen, so that images are displayed thereon.
An electron gun of a CRT contains a cathode for emitting electrons, and a plurality of cylindrical electrodes, namely grids for converging the electrons emitted from the cathode into an electron beam and also for accelerating the electron beam. These grids are called the first grid (G1), the second grid (G2), etc., counting from the cathode side.
The modulation of the electron beams by image signals (namely, to vary the amount of these electron beams reaching the fluorescent screen) is controlled based on the relative potential difference between the first grid and the cathode. Conventionally, a so-called "cathode-drive type system" is generally employed in which the image signals are supplied to the cathode.
Also, in a large-sized projection type television receiver, electron beams are projected from three color CRTs (called "R (R), green (G), blue (B) projection tubes") via a lens toward either a reflection-type screen or a projection-type screen, so that images are displayed on this screen. In this projection-type television receiver, the configuration of the CRTs are similar to those described above.
A description will now be made of a shading phenomenon in a television receiver.
Generally speaking, since the screens (i.e., fluorescent surfaces or screens) of television receivers are made substantially flat, the distances over which the electron beams emitted from an electron gun (or a projection tube) have to reach the screen differ between the central portion of this screen and peripheral portions thereof. As a result, a so-called "shading" phenomenon occurs even for equivalent amounts of electron beams. That is, the further the distance from the central portion of the screen to the peripheral portions thereof, the more the luminance (brightness) of the screen is lowered.
Conventionally, a shading correction circuit such as that shown in FIG. 1 is employed so as to eliminate the brightness inequalities of the fluorescent screens caused by the shading phenomenon. It should be noted that this shading correction circuit indicated in FIG. 1 is employed for each of the R, G, and B image signals.
In FIG. 1, an image signal VIN is compared with a standard voltage V.sub.s corresponding to the black level V.sub.I of the screen by a comparison amplifier (amp) 1, so that only the signal components that exceed the standard voltage V.sub.s are inputted into a multiplier 2. In the multiplier 2, the inputted image signal voltage is modulated based on a correction wave signal such as a saw-tooth wave signal or a parabolic wave signal to thereby calculate a correction voltage V.sub.c.
In this case, the correction voltage V.sub.c is changed linearly from a higher brightness level of the image signals to a lower brightness level thereof. This is because even when a bright screen is properly corrected, overcorrection is made for a dark screen even at the same point on the screen.
It should be understood that the sorts of the above-described correction wave signals are determined by considering the three electron guns (projection tubes) arrangement system, and also the nature of the shading phenomenon resulting from the shape, of the fluorescent surface (or screen).
The correction voltage V.sub.c outputted from the multiplier 2 is added to the image signal voltage V.sub.IN by an adder 3, so that a shading-corrected image signal voltage V.sub.out such as that shown in FIG. 1 is obtained. This shading-corrected image signal voltage V.sub.out is supplied to a cathode K of a CRT 4. As a result, if the same image signal voltage V.sub.IN has been applied, then the brightness of the center portion of the screen is theoretically identical to that of the peripheral portion.
However, in the above-described conventional shading correcting method, the image signal voltage V.sub.out obtained by modulating the image signal voltage V.sub.IN with the correction waveform signal in the multiplier 2 is applied to the cathode of the CRT--in other words, the image signal voltage V.sub.out is directly modulated. Accordingly, the S/N ratio deteriorates, and the brightness of the center portion of the screen must be lowered to eliminate the difference between the brightness of the corner portions and the brightness of the center portion thereof.